1. Field of Invention
The field of the present invention relates in general to modems and line drivers and more particularly reducing power dissipation therein.
2. Description of the Related Art
The North American Integrated Service Digital Network (ISDN) Standard, defined by the American National Standard Institute (ANSI), regulates the protocol of information transmissions over telephone lines. In particular, the ISDN standard regulates the rate at which information can be transmitted and in what format. ISDN allows full duplex digital transmissions of two 64 kilo bit per second data channels. These data rates may easily be achieved over the trunk lines, which connect the telephone companies' central offices. The problem lies in passing these signals across the subscriber line between the central office and the business or residential user. These lines were originally constructed to handle voice traffic in the narrow band between 300 Hz to 3000 Hz at bandwidths equivalent to several kilo baud.
Digital Subscriber Line (DSL) technology and improvements thereon including G.Lite, ADSL, VDSL, HDSL (all of which are broadly identified as X-DSL) have been developed to increase the effective bandwidth of existing subscriber line connections, without requiring the installation of new fiber optic cable. An X-DSL modem operates at frequencies higher than the voice band frequencies, thus an X-DSL modem may operate simultaneously with a voice band modem or a telephone conversation. There are two line codes for modulating for XDSL communications: discrete multi-tone (DMT) and carrierless AM/PM (CAP). Currently, there are over ten discrete XDSL protocols, including: G.Lite, ADSL, VDSL, SDSL, MDSL, RADSL, HDSL, etc.
One of the primary factors limiting the bandwidth or channel capacity of any of the above discussed X-DSL protocols is power. Large amounts of power are required to drive high baud rates across subscriber lines or other wired or wireless communication media. Most of the power in X-DSL systems is consumed in the line driver which drives the modulated signal onto the subscriber line. The line driver may account for 60% or more of the overall system power requirement.
The modulated signal supplied to a line driver typically has a normal power distribution, meaning that the peak-to-average power ratio is relatively high. Thus to avoid clipping or other distortion the occasional high peak signals require a high voltage supply. Efforts have been made to reduce the amount of power consumed in a line driver by use of multiple power levels. Texas Instruments THS6032 Class G amplifier architectures use four power supplies at ±5 and ±15 Volts to reduce power consumption. The ADSL line to which this amplifier is coupled is driven at higher voltage levels only when required. U.S. Pat. No. 6,028,486 entitled “Method and Apparatus for Reducing Power Dissipation in Multi-Carrier Amplifiers” issued on Feb. 22, 2000 and assigned to Telefonaktiebolaget LM Ericsson also teaches apparatus for reducing power in a DSL line driver. In a first embodiment, resembling that employed in the Texas Instruments device, a single amplifier is switchably connected to high and low power supplies, in response to the threshold level of an input signal. In a second embodiment, dual amplifiers, one with a high power supply and the other with a low power supply, are switchably connected to the load in response to the threshold level of an input signal. Both the Texas Instruments and Ericsson designs require an increase in the number of power supplies and/or amplifiers associated with a subscriber line.
What is needed are approaches to power reduction with an improved form factor when compared with the prior art designs.